This invention relates to a cooling air circuit for a gas turbine bucket tip shroud.
Gas turbine buckets have airfoil shaped body portions connected at radially inner ends to root portions and at radially outer ends to tip portions. Some buckets incorporate shrouds at the radially outermost tip, and which cooperate with like shrouds on adjacent buckets to prevent hot gas leakage past the tips and to reduce vibration. The tip shrouds are subject to creep damage, however, due to the combination of high temperature and centrifugally induced bending stresses. In U.S. Pat. No. 5,482,435, there is described a concept for cooling the shroud of a gas turbine bucket, but the cooling design relies on air dedicated to cooling the shroud. Other cooling arrangements for bucket airfoils or fixed nozzle vanes are disclosed in U.S. Pat. Nos. 5,480,281; 5,391,052 and 5,350,277.
This invention utilizes spent cooling air exhausted from the airfoil itself for cooling the associated tip shroud of the bucket. Specifically, the invention seeks to reduce the likelihood of gas turbine tip shroud creep damage while minimizing the cooling flow required for the bucket airfoil and shroud. Thus, the invention proposes the use of air already used for cooling the bucket airfoil, but still at a lower temperature than the gas in the turbine flowpath, for cooing the tip shroud.
In one exemplary embodiment of the invention, leading and trailing groups of cooling holes extend radially outwardly within the airfoil generally along respective leading and trailing edges of the airfoil. Each group of holes communicates with a respective cavity or plenum in the radially outermost portion of the airfoil. Spent cooling air from the radial cooling passages flows into the pair of plenums and then through holes in the tip shroud and exhausted into the hot gas path. These latter holes can extend within the plane of the tip shroud and open along the peripheral edges of the shroud, or at an angle so as to open through the top surface of the shroud.
In a second exemplary embodiment, relatively small film cooling holes are drilled through the radial plenum walls on both the pressure and suction side of the airfoil. These holes open on the underside of the shroud, in the area of the shroud fillets. In a variation of this arrangement, the leading and trailing plenums as described above are connected by an internal connector cavity. Preferably, the majority of the cooling holes open along the pressure and suction side in the leading edge area of the blade, with fewer holes opening in the trailing edge area. Covers are joined to the shroud to close the plenums and one or more metering holes are drilled in the respective covers in order to control the cooling air exhaust.
In a third exemplary embodiment, the individual radial cooling holes within the airfoil are drilled slightly oversize at the tip shroud end. In other words, each cooling hole may be considered to have its own plenum or chamber. Plugs or inserts are joined to the holes to seal the ends of the latter, while shroud cooling holes are drilled directly into the individual plenums and exit either at the top of the shroud or along the underside of the shroud. A metering hole may be required in the various radial cooling hole plugs to insure proper flow distribution.
In its broader aspects, the invention relates to an open cooling circuit for a gas turbine bucket wherein the bucket has an airfoil portion, and a tip shroud, the cooling circuit comprising a plurality of radial cooling holes extending through the airfoil portion and communicating with an enlarged internal area within the tip shroud before exiting the tip shroud such that a cooling medium used to cool the airfoil portion is subsequently used to cool the tip shroud.
In another aspect, the invention relates to an open cooling circuit for a gas turbine airfoil and associated tip shroud comprising a plurality of cooling holes internal to the airfoil and extending in a radially outward direction; a first plenum chamber in an outer radial portion of the airfoil, each of the plurality of holes communicating with the plenum; additional cooling holes in the tip shroud, communicating with the plenum, and exiting through the tip shroud.
In still another aspect, the invention relates to a method of cooling a gas turbine airfoil and associated tip shroud comprising a) providing radial holes in the airfoil and supplying cooling air to the radial holes; b) channeling the cooling air to a plenum in the airfoil; and c) passing the cooling air from the plenum and through the tip shroud.
Additional objects and advantages of the invention will become apparent from the detailed description which follows.